Genetic adaptation was investigated in broilers selected for seven generations on a normal (A) or a low (B) protein diet. Protein and energy metabolism were studied in males from these selected lines fed on a diet of intermediate protein content. All selected birds retained more nitrogen than those studied 10 years previously. There was no difference in nitrogen retention between groups, although relative growth rate of group B birds was higher. Heat productions relative to gross energy intake were 0.38 (group B) and 0.45 (group A). Energy retentions relative to gross energy intake were 0.39 (group B) and 0.35 (group A); the difference being primarily due to higher fat retention in group B. Using a common maintenance requirement for metabolisable energy, group B utilised metabolisable energy for growth (0.78) better than did group A (0.71). At 53 d of age plasma glucose (10%) and insulin (50%) were higher in group B than in group A.